1. Field of Invention
The present invention generally relates to logging systems, especially systems that employ downhole antennas. More specifically, the invention concerns a sealed, modular antenna for measuring electrical characteristics of wellbore fluids and formations. The invention also concerns a method of making such an antenna and an antenna assembly.
2. Description of Related Art
Many downhole operations already use antennas for various purposes. For instance, electrical antennas are employed in downhole wireline logging tools to measure the electrical resistivity of wellbore fluids or formation materials. Additionally, antennas are sometimes employed in measurement-while-drilling ("MWD") and logging-while-drilling ("LWD") applications.
Referring to FIG. 1, downhole antennas have usually taken the form of one or more coils 101 of wire 100, wrapped around a long, narrow, cylindrical mandrel 102. The wire 100 may comprise a length of copper or other suitable conductive material. The ends (not shown) of the wire 100 are usually connected to coaxial cables directly or through hermetic connectors (not shown). The coaxial cables carry electrical signals from the wire 100 to electrical equipment that may be contained within the mandrel 102 or within a section of a downhole string. The mandrel 102 may be part of a drilling string (not shown), a production string, a wireline assembly, or the like. In some applications, wellbore fluids such as drilling mud flow through a central cavity in the mandrel 102. Mandrels are typically made of steel or steel alloys.
The mandrel 102 typically has defined therein a number of annular grooves 104, wherein each groove is wrapped with an insulating, dielectric material 108, such as fiberglass. The coils 101 are wound about the dielectric material 108, and sealed from fluids with an insulating filler 106, such as rubber. The mandrel 102 is sometimes encased by a sleeve 110, which typically comprises a non-conductive, hollow cylindrical sleeve or a slotted metal cylindrical sleeve. The sleeve 110 protects the coils 101 and the filler 106 from damage.
Although many people find the above-mentioned arrangement to be satisfactory for their purposes, this arrangement is not as useful as some might like for their applications. For instance, when the coils 101 are wound around the mandrel 102, they might be inadvertently crossed, thereby electrically shorting one or more coils 101. Another potential problem is that coils 101 of the antenna sometimes electrically short due to moisture invasion, which may occur if wellbore fluids penetrate the sleeve 110 and the filler 106. Moreover, a break or separation may develop in one of the coils 101 under certain circumstances.
These problems cannot be remedied without removing the filler 106, the dielectric material 108, and the wire 100. This is typically a laborious process. In particular, the filler 106 and the dielectric material 108 must be extracted, as well as one or more coils 101. In many cases, the entire wire 100 is removed by a machine and discarded. After the old wire is disconnected from its coaxial cable or hermetic connectors, a new layer of dielectric material is placed around the mandrel, the new wire is connected to the coaxial cable or hermetic connectors, and the coils of the new wire are wound around the new dielectric layer.